There is known an image processing device for determining whether an original image is a color image or an achromatic color image based on read image data. Here, the read image data represents the original image and is obtained by reading the original image by the image reading unit. The image processing device divides the read image into a plurality of blocks and executes a block determining process for determining whether each of the blocks is a chromatic color block or an achromatic color block. More specifically, the image processing device determines whether the number of chromatic color pixels among from all the pixels in the block is greater than or equal to a threshold value or not. Based on the result of this determination, the image processing device determines whether the block is a chromatic color block or an achromatic color block. This determination is made for each of the blocks. The image processing device determines that the original image is a chromatic color image when the ratio of the number of chromatic color blocks to the total number of blocks is greater than or equal to a predetermined ratio.